1. Field of the Invention
The present invention relates to an optical recording medium with which recording/reproducing or reproducing is performed by irradiating a light thereto by means of at least an objective lens having a numerical aperture equal to or more than 1 and relates to its optical recording/reproducing method.
2. Description of the Related Art
The optical recording medium (or a magneto-optical recording medium) represented by a CD (Compact Disc), an MD (Mini Disc) and a DVD (Digital Versatile Disc) is widely utilized as a storage medium of music information, video information, data, programs and so on.
However, based on the demands for a higher sound quality, a higher picture quality and a longer operable time in connection with music information, video information, data, programs and the like, an optical recording medium having a still larger capacity as well as an optical recording/reproducing method and apparatus for recording on and reproducing from such an optical recording medium have been desired.
Thus, to cope with the above-described demands, in the various optical recording/reproducing apparatus, it has been attempted to reduce wavelengths of a light source, for example, a semiconductor laser or to increase a numerical aperture of an objective lens for reducing a diameter of a light spot converged through the objective lens.
For example, as to the semiconductor laser, a GaN semiconductor laser having oscillation wavelengths reduced from 635 nm of the conventional red color laser to 400 nm region has been put into practice, whereby the diameter of the light spot has been reduced.
Further, a so-called near-field optical recording/reproducing system has been examined, in which an objective lens having a numerical aperture of, for example, 1 or more is constituted by using, for example, an optical lens represented by an SIL (Solid Immersion Lens) which has a large numerical aperture and at the same time an objective surface of the objective lens is made to approach to the optical recording medium such that a distance between them may be about a wavelength of the light from a light source or about 1/10 thereof, for recording and reproduction.
As shown in FIG. 21, for example, this near-field optical recording/reproducing system is realized by means of a lens structure of SIL and the like, that is, an objective lens system composed of a first optical lens 21 formed by a high refractive index material and worked, for example, as super-hemispherical shaped and a second optical lens formed by such as a convex lens each positioned in this order from an optical recoding medium 10 side. Various optical recording media and optical recording/reproducing apparatuses which enable the near-field reproduction by using a lens system whose numerical aperture is equal to or more than 1 such as SIL have been proposed in the prior art. (see patent references, for example, 1 and 2 below)
Here, to realize a high-density recording in the near-field optical recording/reproducing system, the same approach is necessary as that of the conventional optical recording/reproducing system and it is necessary to reduce a size of a condensed light spot by shortening the wavelength of a light emitted from a light source and by increasing a numerical aperture of an objective lens.
In this connection, since the area of the condensed light spot is inversely proportional to the square of the numerical aperture of the objective lens, it is effective to increase the numerical aperture of the objective lens in order to realize a high-density recording in the near-field optical recording/reproducing system.
When the optical lens 21 is a super-hemispherical shaped optical lens as, for example, explained in connection with FIG. 21, the numerical aperture NA of the near-field objective lens is expressed as NA=nL1×nL1×sin θAIR supposing that the refractive index of the first optical lens is nL1 and the angle of the incident light relative to the first optical lens through the air is θAIR. (see the Published Document 1 other than Patents below)
Consequently, it becomes necessary to increase the refractive index of the first optical lens in order to increase the numerical aperture of the objective lens.
However, a good reproducing characteristic cannot be obtained even if the refractive index of the first optical lens is increased and the numerical aperture NA of the objective lens is increased, and as a result it becomes difficult to realize a high-density and a large-capacity recording/reproducing.
The followings are related published documents:    [Patent Reference 1]
JP Laid-open Patent Publication No. 2001-522117 (WO9923646)    [Patent Reference 2]
JP Laid-open Patent Publication No. 2001-524246 (WO9849675)    [Published Document 1 other than Patents]    T. D. Milster, “Chromatic Correction of High-Performance Solid Immersion Lens Systems”, Jpn. J. Appl. Phys., March 1999, Vol. 38, Part 1, No. 3B, pp. 1777-1779